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Muslims have begun fasting as the Islamic holy month gets under way.

During Ramadan, Muslims are expected to abstain during daylight hours from food, drink, smoking and sex to focus on spirituality, good deeds and charity. It culminates with the three-day holiday of Eid al-Fitr.

The holy month commemorates the revelation of the Koran to Mohammed and is said to be when the gates of heaven are open, Muslims believe.

The start of Ramadan varies slightly because Muslim countries and groups use different ways of calculating when the new moon crescent is sighted.

Some countries use astronomical calculations and observatories, while others rely on the naked eye alone, leading sometimes to different starting times in the Middle East.

Ramadan started in Thailand and Indonesia before officially beginning in other countries across the world.

The Muslim lunar calendar moves backwards through the seasons, so Ramadan starts 11 days earlier each year under the Western calendar, meaning this year it will clash with the London 2012 Olympics.

However, many of the 3,500 or so Muslim athletes expected to compete at the London Olympics will keep eating as usual.

Egyptian kayaker Mustafa Saied, a practising Muslim, told Associated Press: "I could not fast. I need all that stuff, like protein, carbs and minerals.

"I can do it after Ramadan and Allah will accept it because there was an important reason."

The last time Ramadan started in mid-July was in 1980 during the Moscow Games.

Syria's government, embroiled in the civil war wracking the country, said the holy month would start a day later than the Syrian national council declared.

"The holy month of Ramadan will be the month of victory against the criminals," said a regime spokesman.